1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery and, more particularly, to a nonaqueous electrolyte secondary battery containing an improved electrolyte.
2. Description of the Related Art
In recent years, a nonaqueous electrolyte battery has attracted attention as a high energy density battery. Of such nonaqueous electrolyte batteries, a primary battery using a light metal such as lithium, sodium, or aluminum as a negative electrode active material and manganese dioxide (MnO.sub.2), carbon fluoride [(CF)n], thionyl chloride (SOCl.sub.2), or the like as a positive electrode active material is already widely used as a power source of a timepiece or a backup battery of a memory.
In addition, as the sizes and weights of various types of electronic equipment such as communication equipment have been decreased, a demand for a secondary battery having a high energy density which can be suitably used as a power source of such equipment has been increased, and a nonaqueous electrolyte secondary battery has been actively studied. For example, a nonaqueous electrolyte secondary battery using lithium as a negative electrode and an electrolyte prepared by dissolving an electrolytic salt such as LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, or LiPF.sub.6 in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), .gamma.-butyrolactone (.gamma.-BL), or tetrahydrofuran (THF) has been studied. In addition, a compound which topochemically reacts with lithium such as TiS.sub.2, MoS.sub.2, V.sub.2 O.sub.5, or V.sub.6 O.sub.13 has been studied as a positive electrode material.
The above secondary battery, however, has not been put into practical use yet. This is mainly because a charge/discharge efficiency of the battery is low and its number of charge/discharge times or cycle life is short. The reason for this is assumed that lithium as a negative electrode is degraded due to a reaction with an electrolyte. That is, lithium dissolved in an electrolyte as lithium ions upon discharge reacts with a solvent and its surface is partially deactivated when it precipitates upon charge. Therefore, when charge/discharge is repeated, lithium is precipitated in the form of dendrites or small spheres, or is separated from a collector. Since a combination of an electrolyte constituting an electrolytic salt and a nonaqueous solvent has a large effect on a degree of such degradation in lithium, an optimal combination has been studied.
For example, Japanese Patent Disclosure (Kokai) No. 62-105375 discloses a method of using an electrolyte containing a solvent mixture composed of 10 to 50 vol % of ethylene carbonate and 50 to 90 vol % of 2-methyltetrahydrofuran. In addition, it is known to use electrolytes containing, as a nonaqueous solvent, solvent mixtures of sulfolane and 1,2-dimethoxyethane, sulfolane and tetrahydrofuran, ethylene carbonate and tetrahydrofuran, and ethylene carbonate and propylene carbonate. None of nonaqueous electrolyte secondary batteries using the above nonaqueous electrolytes, however, has achieved a sufficient charge/discharge efficiency.
Elecrtrochem, Acta, 30, 1715 (1985) reports that high lithium charge/discharge can be obtained by using a nonaqueous electrolyte prepared by dissolving 1.5 mol/l of LiAsF.sub.6 as an electrolytic salt in a solvent mixture of ethylene carbonate and 2-methyltetrahydrofuran. LiAsF.sub.6, however, has a problem of toxicity. Therefore, the use of LiPF.sub.6 or LiBF.sub.4 having substantially the same molar conductivity as that of LiAsF.sub.6 has been studied. Since, however, both LiPF.sub.6 and LiBF.sub.4 have problems of poor chemical stability and the like, it is difficult to obtain a sufficient lithium charge/discharge efficiency by using a nonaqueous electrolyte prepared by dissolving an electrolyte of this type.
Japanese Patent Disclosure (Kokai) No. 59-134568 discloses a nonaqueous electrolyte secondary battery using an electrolyte containing a solvent mixture of ethylene carbonate, propylene carbonate, and at least one member selected from the group consisting of 2-methyltetrahydrofuran, 1,2-dimethoxyethane, and 1,3-dioxolane as the third solvent.
Japanese Patent Disclosure (Kokai) No. 63-292578 discloses a technique of improving an electrolyte. This invention describes the use of an electrolyte prepared by dissolving LiPF.sub.6 in 2-methyltetrahydrofuran and electrolyzing the resultant solution by using lithium as an anode. Another known electrolyte improving technique is "THE EFFECT OF DESICCANTS ON THE CYCLING EFFICIENCY OF THE LITHIUM ELECTRODE IN PROPYLENE CARBONATE-BASED ELECTROLYTES" presented by V. R. Koch, et al. in Electrochimice. Acta., Vol. 23, PP. 55-62. This reference discloses an effect of performing a neutral alumina treatment and pre-electrolysis for an electrolyte prepared by dissolving LiAsF.sub.6 in polypropylene carbonate in relation to a lithium cycling efficiency.